1. Field of the Invention
Embodiments of the present invention relate to an internal combustion engine control method. The control method helps to automatically control the fuel injection amount by setting the actual RPM of an internal combustion engine to a target RPM independent of fluctuations in the load of the internal combustion engine.
2. Description of the Related Art
There has been known a vehicle configured to utilize the power of an internal combustion engine for traveling and other operations. For example, a conventional forklift is equipped with an internal combustion engine such as a diesel engine, and utilizes the power of the internal combustion engine for both vehicle movement and the operation of a fork and a mast on which the fork is mounted. In order to facilitate the operation of the vehicle, the vehicle is supplied with an isochronous control function. The isochronous control helps to automatically control the fuel injection amount such that the RPM of the internal combustion engine is an RPM corresponding to the amount of operation by an operator even if the load of the internal combustion engine fluctuates.
In a forklift, for example, an internal combustion engine is operated, and a hydraulic pump is driven during the operation of the engine. The hydraulic pump supplies operating fluid to a hydraulic cylinder serving as an actuator to drive the fork and the mast. The load of the hydraulic pump fluctuates in loading/unloading cargo. In the situation where no isochronous control is employed, the engine load fluctuates due to the load fluctuation in the loading/unloading cargo. Thereby, the RPM of the engine and the RPM of the hydraulic pump are changed.
As a result, the amount of operating fluid is changed, and the speed at which the fork is raised or lowered, for example, is changed. Accordingly, when performing the operation of loading and unloading cargo, the operator must perform the operation while adjusting the amount by which the accelerator pedal is depressed according to the load fluctuation in the loading/unloading cargo.
In the case where isochronous control is adopted, the RPM of the internal combustion engine is automatically controlled so as to be constant (target RPM). Load fluctuation may be generated during loading/unloading of cargo. Isochronous control can be used during load fluctuation whether or not the operator moves the accelerator pedal. Also, when the accelerator pedal is depressed during traveling, the target RPM is determined according to the amount by which the accelerator pedal is depressed independent of whether there is any cargo or not.
In an internal combustion engine, there may be separately adopted special controls for cold starting. In such a control, in order to avoid the unintended stopping of the internal combustion engine (i.e., so-called engine stall) at the time of cold starting, the RPM of the engine is corrected such that the target RPM is higher than the RPM after the warming-up of the internal combustion engine.
Japanese Laid-Open Utility Model Publication No. 6-43237 discloses an idling rotational speed control apparatus for diesel engines. In the control apparatus, the target rotational speed (target RPM) is reduced stepwise with an increase in the temperature of cooling water. This helps to control the idling rotational speed through PID control without involving the generation of unnecessary engine revving.
Japanese Laid-Open Patent Publication No. 2008-82303 discloses a construction-equipment engine control apparatus. In the control apparatus, when an oil temperature detected based on a detection signal from an oil temperature sensor is lower than a predetermined temperature, the engine idling RPM is increased to a predetermined value.
If the related-art technique disclosed in Japanese Laid-Open Utility Model Publication No. 6-43237 or Japanese Laid-Open Patent Publication No. 2008-82303 would be applied to an internal combustion engine adopting isochronous control, the operator might feel discomfort. In this case, there is set a basic target RPM predetermined in correspondence with the depression angle of the accelerator pedal. A correction RPM at the time of warming-up is added to the basic target RPM. When, after warming-up, the engine is restored to the control where no correction RPM is added to the basic target RPM, the operator is caused to feel discomfort.
The related-art technique disclosed in Japanese Laid-Open Utility Model Publication No. 6-43237 or Japanese Laid-Open Patent Publication No. 2008-82303 is indicated by the dotted-line graph Z1 of FIG. 3. As shown in graph Z1, when the accelerator pedal is depressed while adding the correction RPM to the basic target RPM, the RPM of the internal combustion engine gradually increases. At point in time T1, the cooling water temperature or the oil temperature reaches the predetermined temperature and the addition of the correction RPM for setting the target RPM high is canceled. Thus, transition is effected to a control where no correction RPM is added. As a result, at point in time T1, the target RPM corresponding to the depression angle of the accelerator pedal is greatly lowered. Graph K1 of FIG. 3 indicates the locus of the value of the basic target RPM, which increases in correspondence with the depression of the accelerator pedal. Graph K2 indicates the locus of a value obtained by adding the correction RPM (which corresponds to the increase in the target RPM when the cooling water temperature or the oil temperature is lower than the predetermined temperature) to graph K1.
In the related-art control method indicated by graph Z1 of FIG. 3, the RPM of the internal combustion engine gradually increases during the period prior to point in time T1. At point in time T1, the target RPM is greatly lowered despite the depression of the accelerator pedal; after this, the target RPM increases. The actual RPM of the internal combustion engine is controlled so as to be in conformity with the target RPM. Thus, during the depression of the accelerator pedal (i.e., while acceleration is being required by the operator) when an increase in RPM is produced, an unintended abrupt reduction in RPM may occur. Thus, the operator feels discomfort, and may erroneously suppose that some malfunction has been caused in the internal combustion engine. In this case, the operator unnecessarily performs inspection or the like, which may result in in operational inefficiency.
Therefore, there is need in the art for an internal combustion engine control method which allows a change in the target RPM without causing the operator to feel discomfort.